This invention relates an actuator for operating a variator forming part of a transmission, and in particular to combine a harvester comprising such a transmission.
Combine harvesters generally include a threshing drum. In order to provide for the threshing drum to be driven at different speeds a transmission is provided between the engine and the threshing drum. It is desirable to run the combine harvester""s engine at a substantially constant speed, and therefore an infinitely variable transmission has been found to be the most effective transmission for transmitting drive from the engine to the threshing drum. Two types of infinitely variable drives are commonly used in combine harvesters to provide drive for the threshing drum. The first is a belt and pulley drive, at least one of the pulleys being in the form of a variator, where the pulley wheel is separated into two halves known as sheaves. The distance between the sheaves can be adjusted in order to change the effective diameter of the pulley, and hence the ratio of the transmission. The second type of infinitely variable drive used is a hydrostatic transmission. The present invention is concerned with transmissions of the variator type.
The sheaves of the variator are moved towards and away from each other by means of a fluid operated actuator (generally hydraulic fluid). A part of the actuator is connected to the variator for rotation therewith. Another part of the variator is held stationary, and is connected to a supply of hydraulic fluid, which when introduced into or removed from the actuator causes the sheaves of the variator to move towards or away from each other. In a known actuator a rotational rubber seal is used to provide a hydraulic seal between stationary and rotating parts. The rubber seal wears and must be replaced. Replacing the seal causes downtime for the combine harvester, which during harvest may be detrimental to the gathering of the crop.
According to an aspect of the present invention, there is provided a fluid operated actuator including a shaft and a housing. The shaft is at least partially mounted in the housing for relative rotation between the shaft and the housing. The shaft includes a substantially central bore in which a first piston is mounted. The shaft also includes a fluid inlet that is connectable to supply of pressurised fluid and a fluid outlet. A conduit is provided for the flow of fluid between the inlet and the outlet. One end of the housing includes a chamber in which there is mounted a valve assembly comprising a second piston and a valve arranged to selectively open or close an inlet to the chamber. Fluid exiting the outlet opens the valve and exerts a force on the second piston, causing movement thereof in a first direction. Upon movement of the first piston towards the second piston, the first piston engages with the valve to open the inlet to the chamber and allow fluid to flow out of the chamber, generating movement of the second piston in a second direction opposite to the first direction. The invention is also directed to a method by which the described apparatus operates and including method steps for carrying out every function of the apparatus.